What is Anaerobic Digestion?
Anaerobic digestion is a series of biological processes in which microorganisms break down biodegradable material in the absence of oxygen. One of the end products is biogas, which is combusted to generate electricity and heat, or can be processed into renewable natural gas and transportation fuels. A range of anaerobic digestion technologies are converting livestock manure, municipal wastewater solids, food waste, high strength industrial wastewater and residuals, fats, oils and grease (FOG), and various other organic waste streams into biogas, 24 hours a day, 7 days a week. Separated digested solids can be composted, utilized for dairy bedding, directly applied to cropland or converted into other products. Nutrients in the liquid stream are used in agriculture as fertilizer.
The Biological Process
The digestion process begins with bacterial hydrolysis of the input materials in order to break down insoluble organic polymers such as carbohydrates and make them available for other bacteria. Acidogenic bacteria then convert the sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids. Acetogenic bacteria then convert these resulting organic acids into acetic acid, along with additional ammonia, hydrogen, and carbon dioxide. Finally, methanogens convert these products to methane and carbon dioxide.
Many different anaerobic digester systems are commercially available. The following is an overview based on organic waste stream type (manure, municipal wastewater treatment, industrial wastewater treatment and municipal solid waste): Manure: Anaerobic digestion systems for livestock manure operate to reduce methane emissions, odors, pathogens and weed seeds and produce biogas. They fall into four general categories:
Wastewater treatment plants employ anaerobic digesters to break down sewage sludge and eliminate pathogens in wastewater. Technologies available for municipal wastewater fall into tthree general categories — mesophilic, thermophilic, and temperature-phased systems.
Food and beverage manufacturing facilities typically generate high strength waste streams as a by-product of their manufacturing operations. These waste streams are characterized by high Chemical Oxygen Demand (COD) and solids loading, making them well-suited for treatment using anaerobic processes.
Municipal Solid Waste (MSW)
Anaerobic digestion of the organic fraction of MSW provides an engineered and highly controlled process of capturing methane, especially when compared to landfill gas capture of methane generated by putrescible waste. Typically, digestion of mixed solid waste is done as part of compliance with directives to stabilize the organic fraction of the waste stream prior to disposal. The current trend is toward anaerobic digestion of source separated organic waste streams, including food waste, yard trimmings and soiled paper. Presorting is necessary to prevent clogging of the pumps and to reduce the amount of reactor volume occupied by inert material. Even source-separated waste inevitably contains metal and plastic contaminants and must be presorted. Anaerobic digestion systems for MSW include:
How Biogas is Upgraded to Biomethane/Renewable Natural Gas
Captured biogas is transported via pipe from the digester, either directly to a gas use device or to a gas treatment system (e.g. for moisture or hydrogen sulfide removal). If the feedstocks contains high concentrations of sulfur and no steps are taken to prevent formation of hydrogen sulfide in the digester, then hydrogen sulfide is removed from the biogas to prevent corrosion of the combustion device or other downstream equipment.
Captured biogas can also be further upgraded by removing carbon dioxide, nitrogen and oxygen in order to meet the high purity and BTU requirements for pipeline injection or compressed biomethane vehicle fuel.
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